Performing a reliable underwater communication is a relatively complicated task. It is known that electromagnetic waves are significantly attenuated when propagating through water. The only frequency band that is used for electromagnetic underwater communication is the VLF (Very Low Frequency) band, in the range of up to 10 kHz. In this range, high power transmission is needed, and use of extremely long antennas is required at both the receiving and transmitting ends. Therefore, such use is generally limited to submarine communications, and cannot be exploited for personal use. For shorter range underwater communication, conventional systems use ultrasound acoustic transmission, generally in the frequency range of 20 kHz-600 kHz. Unfortunately, however, in the acoustic frequency range, the water as a communication medium provides practically only a relatively narrow bandwidth, which limits the speed of the data transfer through water. The ability to reliably transfer data through water with acoustic waves is further complicated by the different layers of water density, resulting from non-constant speed of sound in water, multipath propagation of the signal, fading, and other environmental disturbances. Furthermore, it is known that the propagation speed of ultrasonic waves in water is significantly lower than the propagation speed of electromagnetic waves in air. Therefore, when it is desired to communicate in water between two apparatuses, of which at least one is not stationary, or moves at a low speed, the Doppler effect adversely affects the signal and the ability to reliably interpret the transmitted data at the receiving apparatus.
It has been found that many conventional types of electromagnetic air communication techniques are unable to overcome the abovementioned problems, which are typical of underwater communications.
Wireless apparatus for carrying out communication in water is known in the art. Such apparatus is used for example in telemetry systems for transferring data that was accumulated during oceanographic researches, or in communication devices for divers. Copending Israeli Patent Application No. 121561, filed on Aug. 18, 1997, by the same applicant herein, discloses an underwater communication apparatus and a communication network for divers. Communication devices for divers are also shown in CA 2,141,619, WO 97/26551, and in U.S. Pat. No. 4,463,452. Other existing apparatus, which is capable of transferring data at a relatively low rate, generally operates in the range of no more than about 600 bits per second, a rate which is in general sufficient for telemetry purposes, but not for other purposes which require a significantly higher rate of data transfer, such as real time voice or picture transmission. For satisfying these requirements, it is desired to provide an underwater modem which is capable of transferring data at a much higher transfer rate, at least in the range of about 4800 bits per second to 9600 bits per second.
Moreover, existing apparatus enables underwater communication between two locations being at a relatively close range, generally in the range of less than 150 meters, and require a direct "line of sight" between the transmitting and receiving devices. Such apparatus does not provide means for carrying out reliable underwater communication between two sites that may be located several kilometers away from one another, and between which there is no "line of sight".
It is an object of the invention to provide a modem which can reliably transfer and receive data at a high rate through water. The term "underwater modem" or simply "modem", when used herein, refers to an apparatus which is capable of transmitting and receiving high-rate data through water, unless otherwise specifically stated. By "high-rate" data transmission it is meant to indicate a band rate of at least 1200 bps, and preferably of at least 4800 bps.
It is still another purpose of the invention to provide an underwater modem which can efficiently overcome fading, multipath, Doppler and environmental disturbances.
It is still another purpose of the invention to provide an underwater modem which can eliminate Doppler distortions of the transmitted signal which are due to movement of the transmitting modem, the receiving modem, or both.
It is still another object of the invention to provide an underwater modem comprising means for correcting errors.
It is another object of the invention to provide means for using the said underwater modem in underwater sound communication.
Other purposes and advantages of the invention will become apparent as the description proceeds.